1. Field of the Invention
The present invention relates to a decomposable composite material, and more particularly, relates to a readily decomposable composite material having excellent gas-barrier properties and consisting of aluminum and a thermoplastic polymer which primarily comprises a lactic acid polymer.
2. Related Art of the Invention
For packaging material, polyethylene, polyethylene terephthalate, polyvinyl chloride, polypropylene, paper, aluminum and composite materials of these materials have conventionally been used. Although some of these packaging materials prepared from these plastics are excellent in gas-barrier properties, these materials have a very slow rate of degradation and thus remain semipermanently when discarded as refuse and buried under the ground. Disposal of these plastics in the ocean causes aesthetic damage of a view or destruction of the living environment of marine organisms.
Paper is degradable in the natural environment while lacking in gas-barrier properties and thus suffers from the disadvantage that the packed contents cannot be stored for a long period. On the other hand, aluminum foil has high gas-barrier properties and low mechanical strength and thus suffers the defect that a package is easily broken in the course of transport and the contents cannot be protected.
On the other hand, polylactic acids and a copolymer of lactic acid and other hydroxycarboxylic acids than lactic acid (generic name of the polylactic acid and the copolymer will hereinafter be referred to simply as a lactic acid base polymer) have been known as thermoplastic polymers having degradability. These polymers can be completely biodegraded within a few months to an year in an animal body. When the polymers are placed in soil or sea water, they start to decompose within a few weeks in a moist environment and totally decompose within several years. Further, final degradation products of these polymers are lactic acid, carbon dioxide and water which are nontoxic to human body.
Polymers of lactic acid are usually prepared from a cyclic dimer of lactic acid which is called lactide, and U.S. Pat. Nos. 1,995,970, 2,362,511, and 2,683,136 have disclosed a polymerization process of lactide. U.S. Pat. Nos. 3,636,956 and 3,797,499 have disclosed a process for copolymerizing lactic acid and glycolic acid. In the copolymerization of lactic acid and other hydroxy carboxylic acid copolymer, lactide and a cyclic ester intermediate, for example glycolide, i.e., a dimer of glycolic acid, are used for polymerization.
The above lactic acid base polymer is degradable, but has relatively high gas permeability and leads to problems when the polymer is used in the fields where gas-barrier properties are required. Consequently, development of composite materials which can be decomposed with ease and are excellent in gas-barrier properties has been strongly desired.